Summary of Work: Using a multifactorial study design we previously showed that the site and severity of infection may fundamentally alter the effects of prophylactic granulocyte colony stimulating factor (G- CSF) in Escherichia coli infected rats. With intravenous E. coli challenge, G-CSF pretreatment increased lethality at all bacterial dosages. However, with intrabronchial and intraperitoneal challenge, G- CSF worsened survival at intermediate bacterial dosages (LD50) but improved survival with high bacterial dosages (LD90). We have continued studies using this multifactorial design to investigate the mechanisms underlying these disparate effects of G-CSF. The results of these studies suggest that the ultimate effects of G-CSF in this model are related to its contribution to microbe and microbial toxin clearance and endogenous inflammatory mediator production. This contribution can be fundamentally altered when the site and severity of infection are changed.